The performance of amplifier devices (including parameters like linearity, gain, efficiency, max. available output power) depends on the load connected to said amplifier. The parameters of these amplifier devices are often optimized for a specific reference load, e.g. 50 ohms. That requires matching inside the amplifier device from the impedance at transistor terminals to the cable connectors (50 ohms). Furthermore it requires matching of the antennae to the connecting cables.
For MRI applications (MRI: Magnetic Resonance Imaging) the matching circuits are typically tuned to a fix impedance so that not all patient loading cases can be covered optimally. A compromise is to optimally match to a large patient loading which requests the highest amount of RF power (RF: radio frequency) for the desired B1-field. In cases with lower coil loading less RF power is used to generate the same B1 field in the coil. Thus a certain level of reflection at the coil due to mismatch can be accepted. This strategy worked successfully for single channel transmit systems. However, with RF shimming with two channels and more the demands change so that the tuning to the strongly loaded situation does not always fit.
This rises the demands of a more flexible architecture so that the amplifier unit (by active, adaptive means) can handle a wider range of loading situations (for different regions in the impedance plane of the smith chart without a degradation of the performance.
The gain, efficiency, thermal stress and power output depends on the impedance seen by the amplifier, which depends on the loading and coupling to other output connection devices (e.g. RF coil ports). As an example for a given mismatch up to VSWR 1:4 (VSWR: voltage standing wave ratio), the maximum available RF output power can vary by a factor of 2. That strongly adds to the costs of the MR system if the amplifier unit needs to be over specified by a factor of −2 to cover all impedances.
If the RF amplifier unit is located close to the RF coil, the impedance matching to the coil loading offers the chance to provide active impedance matching inside the RF amplifier device. By integrating in the impedance management a variable impedance circuit, the system cost of the RF amplifier can be considerably being reduced.
The advantage is that the internal communication and monitoring boards which control the RF amplifier can additionally control the impedance matching of the RF amplifier output. An RF amplifier needs to be robust for standing wave ratio (SWR) of up to 1:6. Optimal gain and efficiency matching needs to include information of the load pull characteristics of the RF amplifier output. For reasons of SAR safety (SAR: Specific Absorption Rate), the whole RF chain has to be calibrated and continuously monitored.
Document US 2013/0285659 A1 shows an amplifier device adapted for an RF coil for MRI applications and a corresponding MRI apparatus. The amplifier device comprises a plurality of amplifier channels, each channel including: (i) an input connection device for connecting an RF signal source; (ii) an output connection device for connecting the RF coil; (iii) an RF amplifier unit; and (iv) an impedance matching network. The matching network matches the impedance by means of adjustable capacitors, wherein the matching is based on a mismatch detection using a reflected signal level from the RF amplifier unit. Further, in the paper ‘An alternative tuning approach to enhance NMR signals’ by D. J. Y Marion and H. Desvaux in JMRI 193(2008)153 157 an experimental set up is shown for an NMR probe where it was found that the modulation of the length of wire between the amplifier and a point (A) before the diodes on the amplifier shows a minimum in the reflected power.